The present invention relates in general to communication systems, and is particularly directed to a low cost communication system that obviates the need for a local oscillator, by employing ON-OFF-KEYED (OOK) modulation that is driven by an associated phase/frequency baseband control signal, such as frequency shift keyed (FSK) modulation, pulse position modulation (PPM), pulse width modulation (PWM) and the like, with a prescribed communications protocol that is effective to provide robust, short range radio communications between battery operated devices.
Radio frequency identification (RFID) and other types of communication systems often require that a short-range communications link (e.g., a link on the order of one inch to one hundred feet) be established between sites/devices. An essential prerequisite for such a link is that communications thereover be reliable in the presence of interference from xe2x80x9cnoisexe2x80x9d sources, such as, but not limited to, transmissions from wireless local area networks and RF emissions from microwave ovens. In addition, it is desirable that such short-range systems enjoy low to moderate data rates, long battery life and very low cost. Unfortunately, the low cost and low power operation requirement typically mandates the use of highly integrated, off the shelf, or custom radio frequency integrated circuits (RFICs); the use of complex or expensive RF or IF filters, that are normally found in radio systems to reject interference, is precluded for cost reasons.
The present invention successfully meets the foregoing objectives by providing a relatively low cost short-range communication system, that eliminates the use of a local oscillator by on-off keying (OOK) an RF carrier signal with a phase/frequency modulated baseband control signal, associated with the data to be transmitted, such as frequency shift keyed (FSK), pulse position modulation (PPM), pulse width modulation (PWM) and the like, in a manner that enjoys very low current consumption and thereby reduces power drain. The transmitter is active (keyed ON and OFF) only during actual communication, so that its current drain is not a significant factor in total battery life. Likewise, the receiver is active for only a small fraction of time. The need for large, complex and expensive filters is minimized by using an amplifier detector type receiver which does not require a costly, power-hungry local oscillator and by using a single IF band-pass filter in active form or passively in the form of a low cost ceramic filter such as those used in inexpensive transistor radios. The digital portion of the system can be realized using low cost, and ultra low power CMOS logic. In the present description the term IF shall be understood to mean the detected OOK pulse train produced by the amplitude detector.
As will be described, OOK RF pulses produced by the transmitter""s OOK modulation scheme are encoded with data representative baseband control signal, the data values of which are represented by phase/frequency information, such as frequency shift keyed (FSK) data, pulse position modulated (PPM) data, pulse width modulated (PWM) data, and the link, with the data being differentially encoded prior to the modulation process. In an FSK embodiment, the output of transmitter is keyed on and off by pulses at two different frequencies, one above and one below the IF center frequency, respectively associated with different logical states. An IF center frequency of 455 KHz may be employed to take advantage of readily available low cost ceramic filters. The transmission period for the transmission of a logical xe2x80x981xe2x80x99 is made the same as that for the transmission of a logical xe2x80x980xe2x80x99 by properly scaling the number of OOK pulses sent and the choice of FSK frequency offsets from the IF center frequency. The OOK-FSK transmitter contains a digital signal processing section comprised of an arrangement of counters and shift registers, that are operative to address and read a system random access memory (RAM) for accessing parallel data and converting this data into OOK modulation pulses representing differentially encoded serial data.
In each the OOK-FSK and OOK-PPM/PWM embodiments, the overall receiver protocol is the same. The receiver first detects incoming data by detecting incoming OOK pulses. The preamble bytes contain a known data pattern. The first byte is over-sampled and run through a correlator to synchronize the receiver. The second byte of the preamble is examined to verify that the beginning of a message is being received. Once the preamble is detected, the receiver proceeds to input the data portion of the message and then verifies the CRC at the end of the message. In addition, each receiver may contain a common received signal detection and preamplification portion to demodulate OOK-FSK and OOK-PPM/PWM transmissions.
The common portion of each FSK and PPM/PWM receiver includes a narrow band antenna, to provide a measure of RF selectivity, and reject out of band interference. The output of antenna is coupled to an AM detector with or without RF preamplification, which recovers the pulse train that was originally transmitted by the OOK transmitter. This pulse train is amplified by a high gain, low noise IF amplifier and band-pass filtered. The band-pass filter""s center frequency is matched to the transmitter""s on/off keying rate. To keep cost low, the band pass filter may be implemented as an active filter within an overall RF integrated circuit. As noted above low cost ceramic or other passive components may be used. The output of the band-pass filter is coupled to one or more limiting amplifier stages to remove amplitude modulation from the recovered pulses. The amplitudes of the recovered pulses may be measured by means of a conventional received signal strength indicator (RSSI) circuit.
The output of the RSSI circuit is compared with a xe2x80x98validxe2x80x99 signal level threshold. The result of this comparison is used to gate the recovered data waveform, to ensure that a sufficient signal level is detected to provide valid data, and thereby prevent potentially false data from being coupled to downstream digital processing circuitry. In the FSK receiver, the recovered IF signal is coupled to a digital tone detector which determines if the OOK-FSK pulse rate is within limits, and to an FM discriminator circuit which demodulates the FSK encoded data, such as a conventional resonant quadrature detector. The output of the FM discriminator varies between two voltages as determined by the OOK rate of the transmitter. This alternating voltage signal is converted to respective logic levels for downstream digital processing.
The digital tone detector comprises an arrangement of counters and comparison logic to validate the received OOK data by verifying that the OOK pulse repetition rate is correct. The digital FSK processing circuitry comprises an arrangement of shift register, counters and control logic that is operative to integrate the received data that has been validated by the received signal strength indicator circuit and FM discriminator circuits, verifies and synchronizes to the preamble, converts the validated data into parallel format, and then writes the recovered data to system RAM.
In an PPM-OOK/PWM-OOK embodiment, the output of the transmitter is keyed on and off by pulses that are shifted in phase by the data, rather than using different frequencies to denote different data values. The OOK pulse rate is maintained at a fixed IF center frequency. Data is encoded by shifting the phase or widths of groups of pulses by a prescribed amount (e.g., 180 degrees for a basic PPM or PWM scheme) from following groups where the data bit is a first logical value, (e.g., xe2x80x981xe2x80x99), and not shifting it where the data bit has a second logical value (e.g., xe2x80x980xe2x80x99). A shift in phase is effected by beginning the next pulse group one half (or one plus a half) OOK periods at the IF center frequency where a 180 degree phase shift is needed. Otherwise, the next pulse group is transmitted one OOK period after the transmission of the last pulse in the previous group. The digital portion of the OOK-PPM/PWM transmitter circuit comprises an arrangement of counters and shift registers, that are operative to address and read the system RAM to obtain parallel data and then convert this data into the OOK modulation pulses representing the differentially encoded serial data.
In the PPM or PWM receiver, since there is no FM demodulator, the recovered IF signal is coupled directly to each of the digital tone detector and RSSI threshold and comparator-gate circuitry, which ensures that a sufficient signal level is present to provide valid data, and prevent potentially false data from being coupled to downstream PPM/PWM digital processing circuitry. The digital portion of the OOK-PPM/PWM receiver is operative to recover phase information in the received encoded data stream. Although the OOK-PPM/PWM receiver""s digital processor is somewhat more complex than the OOK-FSK digital processor, the OOK-PPM/PWM digital processing circuitry effectively performs all of the same tasks as the OOK-FSK processor in addition to decoding the data.
The digital receiver circuitry may operate at a sample clock rate that is at a prescribed fraction of the time between incoming OOK pulses. As will be described, the digital PPM/PWM receiver uses an up/down counter mode of operation to provide reliable data detection in environments where some input pulses will be corrupted. The circuit compares the phase of each bit of the received data validated by the received signal strength indicator circuit and discriminator circuitry. It then verifies and synchronizes to the preamble, converts the data to parallel and writes the recovered data to system RAM.